A number of regulatory reactions such as chromatolysis after axon injury, retrograde transneuronal changes, acute glial reactions and growth regulation according to the size of the peripheral field imply the existence in intracellular communication from the axons and/or terminals to the parent cell nucleus. Retrograde intraxonal transport may provide the means for this communication. We propose to study the retrograde intraaxonal transport of exogenous proteins such as horseradish peroxidase from the region of the axon terminal to the neuronal cell body in the amphibian, avian and mammalian nervous systems. We shall analyze by light and electron microscopy and biochemical techniques: 1) the cytological organelles involved in the transport, 2) the development of the transport phenomenon in embryonic stages of the chick, 3) the effects of injury or of alteration in the physiological state of the neuron on the retrograde transport of proteins, 4) the possible interaction of anterograde transported protein with the loading mechanism that preceeds retrograde transport of exogenous macromolecules, and 5) the possiblility of applying additional markers to the further refinement of the retrograde transport technique as a tool for studying neuronal connectivity in the nervous system. From our relatively elementary observations about retrograde transport, we hope to provide basic research information about mechanisms at play in the plasticity of synaptogenesis and of regeneration.